Welding studs of the specified type are frequently used for automated assembly processes. In these processes, the welded-on stud is populated, for example during final assembly in the automaking process, with nuts or other fasteners to which torque is applied by automated screwdriving systems, pneumatic screwdrivers or battery-operated screwdrivers. It is frequently the case that the nut is not correctly placed on the stud. Reasons for this include the short production time available and the spatial conditions in the bare chassis. These circumstances have the result that the nut or stud does not correctly engage the thread, and cross-threading occurs. This destroys the thread, and the specified ultimate strength of the threaded connection cannot be achieved. The result is a repair that entails additional costs.
Solution approaches are known from screw technology, since automated assembly processes have existed in this field for a very long time. Thus, for example, EP 0 345 373 B1, U.S. Pat. Nos. 5,073,073, 4,981,406, 5,609,455, and EP 0 840 859 B1 show options for avoiding cross-threading.
EP 0 345 373, U.S. Pat. Nos. 5,073,073, and 4,981,406 disclose screws that use the principle of an eccentric guide point. The basis of the approach here is that the screw can initially be inserted relatively far into the female thread and can immediately be brought into engagement with the correct threads. This is possible because the eccentricity or conicity of the guide points results in a smaller diameter and makes more lateral space available. After a few rotations, a secure thread engagement is achieved and the screwing process can be continued without damaging the threads.
U.S. Pat. No. 5,609,455 shows a screw with a guide point that initially has no thread at its forward-most end, and in its further progression carries a partial thread that then transitions into a full thread. Here, too, the principle in action is that the guide point can be pushed relatively far into the female thread before thread engagement takes place. One rotation of the screw permits further penetration and further alignment of the screw axis, so that the angle is significantly smaller than the pitch angle of the thread. This serves to significantly reduce the risk of cross-threading.
The screw disclosed in EP 1 008 770 A2 uses two different thread profiles that are intended to achieve better guidance during screwing-in. Moreover, the screw has a guide point that is entirely without threads. The guide point transitions to a region having a round thread that is designed such that the round thread engages the female thread even when the screw is applied to the female thread at an angle. If cross-threading takes place, the engagement of the round thread is not strong enough to destroy the female thread. Instead, the round thread snaps out of the female thread and orients itself in the correct thread. As the screw rotates further, more and more threads become engaged, until finally the fully formed thread at the back end of the screw is engaged and the ultimate strength of the threaded connection is achieved.
The aforementioned approaches are only suitable for threaded fastening processes in which the screw is arranged more or less movably in a driving tool. This is the only way it is possible for the screw to align itself after correct engagement of the threads and for the threaded fastening process to continue without damage. However, welding studs experience a rigid connection to the base material or the base sheet metal. Accordingly, they are often not movable and cannot execute this alignment motion.